From the introduction:
One day, the tentacles attacked.
They seemed to be able to appear anywhere, at any time. An extradimensional horror stirred by modern scientific experiments? Humanity's collective self-loathing psychically manifesting itself? Some sort of spore only now entering a new, fast-growing phase of its eons-long life cycle? Who knows? The explanation isn't important. What's more important is your immediate goal: survival.
Tentacle Attack is an RPG in which the most of the players play characters trying to survive a modern-day tentacle apocalypse. One players is the GM, who plays almost everything else. The only exception is the tentacles. ANY player can call down a tentacle attack anywhere, at any time, on anyone, for any reason.